The biochemical properties (antitumor, carcinogenic and cardiotoxicity) of the anthracyclines may be related to DNA binding and/or the production of oxygen derived toxic free radical metabolites. We have examined the formation of O2- from these drugs and have evaluated the role of oxygen derived toxic species in their biological properties. Our findings indicate that formation of O2- is not related to the antitumor activity. However, lipid peroxidation caused by O2- or OH. is related to the cardiotoxicity of the anthracyclines.